Reproduction in Domestic Animals

62 DJ Skarzynski, G Ferreira-Dias and K Okudato a luteolytic dose of aPGF 2a and suggested that thisincrease in blood flow is related to early luteolytic events(Acosta et al. 2002; Miyamoto et al. 2005). This increasein blood flow is thought to be caused by well-knownvasodilators, PGE 2 and ⁄ or NO (Miyamoto et al. 2005).In support, it has been recently shown that intralutealapplication of a NO donor drastically increase lutealblood flow (Sasahara et al. 2007).However, CL blood flow was reduced 8 h after oncePGF 2a i.m. injection in cows (Acosta et al. 2002) andwithin 4 h after two injections of PGF 2a (at 0 and 4 h)into the uterine lumen in ewes (Nett et al. 1976). Thus,PGF 2a may cause CL regression by depriving the gland ofnutrients, substrates for steroidogenesis, and luteotropicsupport (Nett et al. 1976; Niswender et al. 2000). Therefore,we hypothesized that hypoxic (low oxygen) conditionsin the CL induced by a decreased blood supply isrelated to the luteolytic cascade in cows. We examinedthe influence of hypoxia on the luteal P4 generatingsystem and luteal cell apoptosis using bovine luteal cellculture, and found that hypoxia decreased P4 synthesisby suppressing a steroidogenic enzyme P450scc activity(Nishimura et al. 2007) and also induced apoptotic celldeath by enhancing the expression of pro-apoptoticprotein BNIP3 and by activating caspase-3 (Nishimuraet al. 2008). Since P4 treatment under hypoxia decreasedluteal cell death, the induction of apoptosis by hypoxiaseems to be partially caused by a decrease in P4production (Nishimura et al. 2008). However, in ewesafter a single PGF 2a injection, levels of P4 decreased by12 h after PGF 2a , but total ovarian blood flow did notdecreased significantly until 30 h (Nett et al. 1976).Blood flow to ovine CL and luteal LH receptors didnot decrease until P4 declined in the peripheral blood andfunctional lutelysis was almost completed (Nett et al.1976; Niswender et al. 1976). Moreover, Watanebe et al.(2006) showed that P4 decreased even when blood flowremained high in bovine CL. Therefore, oxygen deficiencydue to decreased blood flow is not mandatory forthe induction of both functional and structural luteolysisin cows, but may play such a supporting role in the finalsteps of bovine CL regression.Resistance to PGF 2a Action During the LutealPhaseThe newly formed CL is resistant to exogenous PGF 2atreatment and action (Henricks et al. 1974; Beal et al.1980). Moreover, the sensitivity of CL to the luteolyticaction of extragonadal PGF 2a seems to increase progressivelytoward the end of the luteal phase (Skarzynskiet al. 1997; Pate 2003; Meidan et al. 2005). Since a singleclass of high-affinity PGF 2a receptors is present withinthe bovine CL by the second day after ovulation(Sakamoto et al. 1995; Wiltbank et al. 1995), neitherthe lack of responsiveness to PGF 2a in the early CL(Henricks et al. 1974; Beal et al. 1980) nor the lowersensitivity to PGF 2a in the mid-luteal CL (Skarzynskiet al. 1997) can be attributed to a deficiency of highaffinityPGF 2a receptors. Wiltbank et al. (1990) suggestedthat incomplete vascularization or incompletedifferentiation of degenerative mechanisms in the earlybovine CL are responsible for the lack of luteolyticcapacity. In recent years, Meidan et al. have explainedhow the products of endothelial cells are involved in theacquisition of luteolytic capacity by the bovine CL(Girsh et al. 1996; Mamluk et al. 1999; Meidan et al.1999, 2005; Levy et al. 2000). They showed that the lackof ET-1 synthesis and response to EDN-1 in the earlyand mid-luteal CL may make the CL unresponsive tothe luteolytic action of PGF 2a (Mamluk et al. 1999;Levy et al. 2000; Rosiansky-Sultan et al. 2006). In fact,PGF 2a upregulated the amount of mRNA encodingEDN-1 converting enzymes during the mid- and latelutealphase of the bovine CL and this action of PGF2awas cycle-phase specific, because it was not observed inthe early CL. Additionally, the steroidogenic cells of theCL may also possess autocrine mechanisms that protectbovine CL from premature luteolysis. PGF 2a was foundto induce mRNA for PTGS-2 on day 11 but not on day4 of the oestrous cycle, suggesting that such autoamplificationof luteal PGF 2a is also a key component ofluteolytic capacity by the bovine CL (Tsai and Wiltbank1998). Moreover, Silva et al. (2000) showed that upregulationof 15-hydroxyPG dehydrogenases in ovine CLduring early pregnancy may deactivate PGF 2a as amechanism for maternal recognition of pregnancy.We showed that the lack of response to PGF 2a by theearly bovine CL and the lower reaction of bovine CL toPGF 2a during the mid-luteal phase depended uponlocally produced PGs, OT and P4 (Skarzynski andOkuda 1999), NA and NO (Skarzynski et al. 2000). Thelack of response to PGF 2a in the early CL could be aconsequence of receptor desensitization and the biologicalwanes over time (Lohse 1993; Skarzynski et al.2001). Based on our findings, one could assume thatluteal neuropeptide, NO, OT, PGs and P4 are componentsof an auto ⁄ paracrine positive feedback cascade inbovine CL, and that they also play roles in regulatingthe function of PGF 2a receptors and the PGF 2a -intracellularcalcium ([Ca 2+ ] i )-protein kinase C cascade(Skarzynski and Okuda 1999; Skarzynski et al. 2000,2001). The auto ⁄ paracrine positive feedback loop can bea mechanism for protection against premature luteolysisduring the early and mid-luteal phase. Furthermore,each of the factors that directly affect PGs, OT and P4secretion by luteal cells may be indirectly involved inregulating function of PGF 2a receptors, and may beresponsible for the varying sensitivity of bovine lutealcells to PGF 2a . In contrast, locally produced NO maysensitize bovine CL to luteolytic action of PGF 2a(Skarzynski et al. 2000, 2003b). The amplified effectsof PGF 2a on luteal cells by pre-exposure to a NO donor(S-NAP) suggest that priming of bovine CL by NO isneeded for complete luteal regression (Skarzynski et al.2000). In support, NO up-regulated PGF 2a receptors inbovine CL endothelial cells (Rosiansky-Sultan et al.2006). Based on the overall findings, the differentresponse by CL to PGF 2a during the luteal phase andpregnancy should be explained not only by changes inthe concentration of PGF 2a receptors but also bychanges in their sensitivity as well as on the maturationof ET-1 and NO systems. Therefore, a number ofdifferent pathways may account for the lack of PGF 2a -induced luteolysis during the early luteal phase as well asduring early pregnancy.Ó 2008 The Authors. Journal compilation Ó 2008 Blackwell Verlag